IC RESPONSE IN PLANTS--BLOCK METHOD

Method of block--Advantages of block method--Plant response a
physiological phenomenon--Abolition of response by anaesthetics and
poisons--Abolition of response when plant is killed by hot water.

I shall now proceed to describe another and independent method which I
devised for obtaining plant response. It has the advantage of offering
us a complementary means of verifying the results found by the method of
negative variation. As it is also, in itself, for reasons which will be
shown later, a more perfect mode of inquiry, it enables us to
investigate problems which would otherwise have been difficult to
attempt.

When electrolytic contacts are made on the uninjured surfaces of the
stalk at A and B, the two points, being practically similar in every
way, are iso-electric, and little or no current will flow in the
galvanometer. If now the whole stalk be uniformly stimulated, and if
both ends A and B be equally excited at the same moment, it is clear
that there will still be no responsive current, owing to balancing
action at the two ends. This difficulty as regards the obtaining of
response was overcome in the method of negative variation, where the
excitability of one end was depressed by chemical reagents or injury, or
abolished by excessive temperature. On stimulating the stalk there was
produced a greater excitation at A than at B, and a current of action
was then observed to flow in the stalk from the more excited A to the
less excited B (fig. 6).

But we can cause this differential action to become evident by another
means. For example, if we produce a block, by clamping at C between A
and B (fig. 14, _a_), so that the disturbance made at A by tapping or
vibration is prevented from reaching B, we shall then have A thrown into
a relatively greater excitatory condition than B. It will now be found
that a current of action flows in the stalk from A to B, that is to say,
from the excited to the less excited. When the B end is stimulated,
there will be a reverse current (fig. 14, _b_).

[Illustration: FIG. 14.--THE METHOD OF BLOCK
(_a_) The plant is clamped at C, between A and B.
(_b_) Responses obtained by alternately stimulating the two ends.
Stimulation of A produces upward response; of B gives downward
response.]

We have in this method a great advantage over that of negative
variation, for we can always verify any set of results by making
corroborat